Trend graph display system

ABSTRACT

A cathode ray tube having a viewing screen on which time points represented by a plurality of points spaced from one another with a predetermined time interval are displayed in the raster direction, while display quantities of trend graphs are displayed as functions of the time points in the direction perpendicular to the raster direction by a plurality of points spaced from one another with a predetermined interval. A raster scanning is effected by sweeping the viewing screen with a scanning point whereby the scanning lines thus produced are correlated to the spaced points representing the display quantities. Apparatus stores the display quantities of the trend graph corresponding to the time points, sequentially reads out the display quantities from the memory apparatus in the order of the time points, and compares the display quantities read out from the memory apparatus with a display quantity represented by an instant scan line. The comparison apparatus compares the display quantity read out corresponding to an instant time point and an instant position of the scanning spot of the instant scan line with a display quantity represented by an instant scan line, supplies a video signal to the cathode ray tube when the instant display quantity represented by the instant scan lines lies in a range delimited by the display quantity at the instant time point and the display quantity at the select one time point of the time points immediately preceding and following the instant time point.

The present invention relates to a trend graph display system. Inparticular, the invention concerns a trend graph display apparatus of araster scan type which serves to display a variation of a physicalquantity as a function of time and in which the time is taken in theraster direction, i.e. along the abscissa, and in which the physicalquantity to be displayed is taken in the direction perpendicular to theraster, i.e. along the ordinate.

The display apparatus using a cathode ray tube (hereinafter referred tosimply as CRT) is widely employed as one of the most convenient meansfor man-machine communication in the electronic computer systems. Amongsuch CRT display apparatus, there has been known a display apparatuswhich serves to display how a particular physical quantity varies as afunction of time, i.e. a so-called trend graph.

The conventional CRT display apparatus incorporates therein a deflectionsystem of a raster scanning type which is effective to sweep or scan theviewing screen in the horizontal direction starting from the upper leftcorner of the screen with the horizontal line scanning sequentiallyshifted downwardly thereby to generate a raster, as is common in thetelevision receivers. In this connection, since the control circuit forthe raster scanning can be implemented in a much facilitated manner whenthe time axis is taken along the ordinate, namely in the directionperpendicular to the scanning or sweeping direction, various approacheshave long been made in this sense. However, for the observation of thegraph display as generated, it is more natural and convenient to takethe time axis along the abscissa, i.e. in the direction corresponding tothe scanning direction. Recent progress in the technology ofsemiconductor memories has stimulated the attempt to implement the trendgraph display with the time elapse taken along the abscissa.

For example, an approach is disclosed in Japanese Patent Applicationfiled Dec. 25, 1971 in the name of Mitsubishi Electric Corporation underthe title "Display System Of Trend Graph" and issued Dec. 23, 1976 asPublication No. 51-48862. According to the disclosure, time pointsrepresented by a number of spaced points are taken along the abscissa onthe display screen of CRT, while physical quantity of a trend graph tobe displayed is taken along the ordinate under representation by anumber of spaced points, whereby the correspondence of the spaced pointsrepresenting the physical quantity to the scan lines has beenaccomplished. To this end, values or magnitudes of the physical quantityof a trend graph to be displayed at every time point are stored in amemory, and the value of the physical quantity at a time pointcorresponding to the instantaneous or present scan point on the displayscreen is read out from the memory to be compared with the physicalquantity represented by an instant scan line. When coincidence is found,a coincidence signal is superposed on the video signal of CRT to producea bright point or spot at the instantaneous scan point on a scan line.

The trend graph display apparatus of the arrangement outlined above ishowever disadvantageous in that image or visual quality tends to bedegraded since the display of a physical quantity in a form of discretepoints on the scan lines is effected in such a manner that only onebright spot is displayed at each time point. For example, when the slopeof the trend graph becomes steeper, distance between the adjacent twospots is also increased and is found to be uncomfortable for observationand the relations among the spots tends to be more ambiguous, therebydeteriorating the continuity of the trend graph.

Another trend graph display system is disclosed, for example, in U.S.patent application Ser. No. 103,741 entitled "Historical Data Display",filed Jan. 4, 1971 by Bunker et al and issued Jan. 12, 1973 as U.S. Pat.No. 3,739,369 assigned now to General Electric Company. According tothis prior art, a value of a physical quantity at a display time pointcorresponding to the instant scan point of a scan line on a CRT screenis read out from a memory and a bright spot is produced at the positionof the instant scan point on the scan line, provided that the value ofphysical quantity represented by an instant scan line is not greaterthan the value of the physical quantity at the instant time point readfrom the memory. In this display system, the region confined between thetrend line as displayed and the base line or abscissa is shadowedthrough shading. Consequently, continuity among the spots will not belost, even when the slope of the graph becomes steeper. However, thegraph as displayed is not in the form of a line or curve but isrepresented as area. In other words, desired curved line representationcan not be attained.

Further, another approach is disclosed in U.S. patent application Ser.No. 88,924 under the title "Circuit Arrangement For The Presentation OfWaveforms On Viewing Screens Utilizing Raster Deflection" filed Nov. 12,1970 in the name of Blixt et al and issued Aug. 22, 1972 as U.S. Pat.No. 3,686,662 assigned now to International Standard ElectricCorporation. According to the disclosure, display time pointsrepresented by a number of spaced points are taken along the abscissa ofa viewing screen of CRT, while physical quantity of the trend graph isdisplayed by a number of spaced points along the ordinate of the viewingscreen, wherein the spaced points representing the physical quantity arecorrespondingly correlated to the scan lines. The physical quantity tobe displayed is provided for every predetermined number of display timepoints, e.g. every eighth time point and stored in a memory. When thephysical quantity at the predetermined time point as stored in thememory coincides with the physical quantity represented by the instantscan line, a bright spot is generated at the corresponding scan pointwith the physical quantity being interpolated at each of the seven timepoints between the adjacent predetermined time points. This displaysystem thus requires a complicated circuit arrangement forinterpolation. Because the interpolation is made between the adjacentpredetermined time points, it becomes difficult to attain a graphicpresentation with a desired accuracy, particularly when the physicalquantity to be displayed as the trend graph undergoes rapid and steepvariations.

Accordingly, an object of the invention is to eliminate the drawbacks ofthe hitherto known trend graph display systems described above.

The above and other objects, features and advantages of the inventionwill be more apparent from the following description with reference tothe drawings, in which:

FIG. 1 shows an example of graphic representation on a viewing screen ofa CRT trend graph display system according to the present invention;

FIG. 2 is a block diagram showing a typical example of the graphicdisplay system to which the present invention is to be applied;

FIG. 3 is a block diagram showing an exemplary embodiment of a trendgraph adapter according to the invention;

FIGS. 4A and 4B illustrate exemplary embodiments of bright point controllogic circuits which can be employed in the trend graph adapteraccording to the invention;

FIG. 5 illustrates an example of trend graph presentation which can beproduced by the trend graph adapter according to the invention;

FIGS. 6A, 6B, 6C and 6D show other examples of the trend graphs producedby the trend graph adapter according to the invention, and

FIG. 7 is a block diagram showing another exemplary embodiment of thetrend graph adapter according to the invention.

The invention will be described in conjunction with preferredembodiments shown, only by way of example, in the drawings.

Referring to FIG. 1 which illustrates examples of the trend graph imageson a viewing screen of a CRT display apparatus according to theinvention, time is taken along the abscissa, while physical quantitiesare taken along the ordinate as functions of time as represented bytrend lines or curves A, B and C.

FIG. 2 shows in a block diagram a typical example of the graphic displaysystem to which the invention can be applied. Referring to the figure,numeral 200 denotes an interface adapter which serves to connect anexternal central processing unit (CPU) 100 or other external informationor data sources to a micro-processor 300 through data bus. Trend graphvideo signals 501l to 501n respectively available from trend graphadapters 500l to 500n and character/symbol video signal 601 availablefrom a character/symbol adapter 600 are ORed through a video controller700, the output video signal 701 from which is applied to CRT 800thereby displaying graph and characters etc. on the screen thereof. Thetrend graph adapter assembly comprises a plurality of the individualtrend graph adapters 500l to 500n in number corresponding to the numberof trend graph lines to be displayed. The interface adapter 200functions to store the character/symbol data from CPU 100 in thecharacter/symbol adapter in a distributed manner and at the same timeoperate to store the trend graph data in the trend adapters 500l to 500nin a distributed manner. The data stored in the trend graph adapters areread out sequentially and output as the trend graph video signals 501lto 501n under the control of dot timing signal 401 and scanning linenumber signal 402 from a timing controller 400 of a conventional type.The data stored in the character/symbol adapter are sequentially readout in response to the dot timing signal 401, a character/symbol timingsignal 403, a raster signal 402 and a line signal 405 and output as thecharacter/symbol video signal 601. The timing controller 400 comprisesan oscillator and frequency dividers, whereby the dot timing signal isproduced through frequency division of the output frequency of theoscillator, the character/symbol timing signal is produced throughcorresponding frequency division of the dot timing signal, the rastersignal is produced through frequency division of the character/symboltiming signal, the line signal is produced through frequency division ofthe raster signal and finally vertical synchronizing signal is producedthrough frequency division of the line signal. CRT 800 responds to theraster signal 405 and vertical synchronizing signal 406 from the timingcontroller and the video signal 701 from the video controller togenerate displays of characters and/or symbols as well as graphic trendcurves on the viewing screen.

FIG. 3 shows in a block diagram an exemplary embodiment of the trendgraph adapter according to the invention. Since the circuit arrangementof the trend graph adapters 500l to 500n shown in FIG. 2 are identicalwith one another, description will be made of the adapter 500n, by wayof example. Referring to FIG. 3, the trend graph adapter 500n comprisesa trend data memory 510n, a first comparator circuit 530n, a secondcomparator circuit 540n, a data latch circuit 550n and a bright spotcontrol logic circuit 560n. The trend data memory 510 may be constitutedby a random access memory or a circulating type memory such as shiftregisters. Data representing a physical quantity to be displayed atindividual display time points taken along the abscissa of the viewingscreen are stored in the trend data memory 510 from CPU 100 through theinterface adapter 200 in the sequence corresponding to the order of thedisplay time points. Assuming for example that the resolving power isrepresented by 256 dots along the abscissa of the display screen (i.e.number of time points is equal to 256) and that the resolution along theordinate is represented by dot number of 256 (i.e. the number ofscanning lines is equal to 256), the physical quantity of the trendcurve at every display time point is converted into a display quantityof a word length of 8 bits through the micro-processor 300 and stored inthe memory 510n. For example, referring to FIG. 5, when a physicalquantity having magnitudes variable in the range of 0 to 60 is to bedisplayed as a trend curve on the screen in the region covering thescanning lines numbered n to n+15, the magnitudes 0 to 60 of thephysical quantity are converted in terms of the numbers of scanninglines which are then used as the quantities to be displayed.Accordingly, the magnitudes 0 and 60 of the physical quantities arerepresented by the numbers of the scanning lines n+15 and n in terms ofthe corresponding 8-bit words, respectively. Although the scanning linesare shown as numbered in the increasing direction from the top towardthe bottom of the screen, it is of course possible to allot the numberto the scanning line in the reverse direction. In this manner, 8-bitwords each representing a magnitude of a physical quantity at a displaytime point are stored in the sequence corresponding to the predeterminedorder of the display time points in the memory 510n which is thusrequired to have a storage capacity of 256 words each consisting of 8bits, i.e. storage capacity of 256 bytes. The contents in the memory510n is read out sequentially in the order of the associated displaytime point on the one-word basis under the control of the dot timingsignal 401 from the timing controller 400 and supplied to the latchcircuit 550n and the input terminal E of the first comparator circuit530n. It is to be noted that the display time point for the magnitude ofthe physical quantity as read out from the memory 510n is the very timepoint that corresponds to the instant position of the scanning spot onthe CRT screen. Accordingly, the read-out of the quantity from thememory is effected at every display time point which corresponds to theposition of every scanning spot on each of the scanning lines on theviewing screen. In the above assumed case, 256 dot timing pulses areproduced during a single line scan period. The latch circuit 550nresponds to the dot timing signal 401 applied to the trigger terminal Tthereof to fetch therein the display data supplied to the data inputterminal D and then responds to the succeeding dot timing signal 401 totransfer the fetched data from the output terminal Q to the inputterminal E' of the second comparator circuit 540 n. In this manner, thelatch circuit 550n serves to delay the output data from the memory 510nfor a single display time interval which is a time span between the twoadjacent display time points (i.e. the quiescent period of the dottiming pulses). The signal 402 representing the number of the scanningline is applied from the timing controller 400 to the input terminals Fof the first and the second comparators 530n and 540n. This signal 402represents the number allotted to the instant scanning line on thescreen in a binary code of 8 bits. Accordingly, the scanning line numbersignal 402 continues to be present at the input terminals E' of thecomparators 530n and 540n so long as the associated line is beingscanned, i.e. during the duration from the time point t₀ to t₂₅₅. Suchscanning line number signal 402 may be produced by a counter provided inthe timing controller 400 adapted to count upwardly the raster signalsand to be reset by the vertical synchronizing signal. In the case wherethe scanning line number is allotted in the decreasing order toward thebottom of the screen, the counter may be of course constituted by a downcounter.

During the single line scanning period in which the scan line numbersignal 402 is held, data of the quantity to be displayed are read outfrom the trend data memory 510n in the sequence corresponding to theorder of the display time points and subjected to comparison through thecomparator circuits 530n and 540n. More specifically, the firstcomparator circuit 530n compares the 8-bit signal representing the scanline number (signal 402) with the data quantity of the trend graph to bedisplayed at the instant display time point, while the second comparatorcircuit 540n compares the scan line number signal 402 with the trenddata at the time point which immediately preceded the instant time pointfor the single display time interval described above.

The first comparator circuit 530n compares the input data at the inputterminals E and F and produces outputs at the output terminals G₁, G₂and G₃, respectively, when the conditions defined below are fulfilled.In particular, when the data of the physical quantity read out from thememory 510n and applied to the input terminal E is represented by E,while the 8-bit data representing the scan line number applied to theinput terminal F is represented by F, output signals (i.e. binarysignals "1") are produced from the output terminals G₁, G₂ and G₃ whenthe following conditions are satisfied:

    G.sub.1 :E<F,G.sub.2 :E=F, and G.sub.3 :E>F

On the other hand, the second comparator circuit 540n serves to comparethe input data applied to the input terminals E' and F. When the displaydata of the physical quantity applied to the input terminal E' isrepresented by E', while the data applied to the terminal F isrepresented by F, there are produced output signals (i.e. binary signals"1") at respective output terminals H₁, H₂ and H₃ when the followingconditions are fulfilled:

    H.sub.1 :E'<F,H.sub.2 :E'=F, and H.sub.3 :E'>F

The circuit arrangement of the comparators for implementing thecomparison functions described above will readily occur to those skilledin the art. One group of the input terminals I₁, I₂ and I₃ of the brightspot control logic circuit 560n are connected to the output terminalsG₁, G₂ and G₃, respectively, of the first comparator circuit 530n, whilethe other group of input terminals J₁, J₂ and J₃ of the control logiccircuit 560n are connected to the output terminals H₁, H₂ and H₃ of thesecond comparator circuit 540n. The bright spot control logic circuit isso arranged that the video signal 501n is produced at the outputterminal K when the input signals I₁, I₂, I₃, J₁, J₂ and J₃ respectivelyapplied to the input terminals I₁, I₂, I₃, J₁, J₂ and J₃ satisfy thefollowing condition:

    K=I.sub.1 ×J.sub.3 +I.sub.2 +I.sub.3 ×J.sub.1  (1)

In other words, the bright spot control logic circuit 560n produces thevideo signals 501n at the individual display time points thereby toproduce the bright spots at the associated scan points, when at leastone of the following conditions is met; namely,

(a) when the coincidence occurs between the quantity to be displayed atthe instant time point as read out from the trend data memory and theinstant scan line number value (i.e. when the signal "1" is present atthe terminal I₂),

(b) when the instant scan line number value is greater than the displayquantity at the instant time point and smaller than the display quantityat the immediately preceding time point (i.e. when the input signal "1"is present at both of the input terminals I₁ and J₂), and

(c) when the instant scan line number value is smaller than the displayquantity at the instant time point and greater than the display quantityat the immediately preceding time point (i.e. when both of the inputterminals I₃ and J₁ are applied with the input signals "1").

Referring to FIG. 6A, assuming for example that the display quantity isequal to n at the instant display time point t₁ while the displayquantity at the immediately preceding time point t₀ is equal to n+4, thebright spots will be produced not only at the end point a where the scanline number coincides with the instant display quantity but also at thepoints b on the scanning lines where the scan line numbers are largerthan the instant display quantity and smaller than the display quantityat the immediately preceding time point t₀. In a similar manner, whenthe display quantity at the instant time point t₁ is equal to n+3 withthe display quantity at the immediately preceding time point t₀ beingequal to n, for example, then the bright spots will be produced not onlyat the end point a but at the scan points C on the scanning linesnumbered n+1 and n+2, as shown in FIG. 6B.

In this manner, even when the trend data undergoes a rapid variation,clear display of the trend curve can be accomplished because of theinterpolation of the bright spots on the intermediate scanning line(s),as described above and also illustrated in FIG. 5 by lines l and m.

In the foregoing description concerning the exemplary embodiment shownin FIG. 3, it has been assumed that the bright spot control is effectedin accordance with the conditions defined by the expression (1) so thatthe bright spot is produced in dependence on the instant displayquantity stored in the trend data memory 510n. However, such control maybe effected under the logic conditions that K=I₁ ×J₃ +I₂ +J₂ +I₃ ×J₁, ifdesired. In this case, the bright spot will be produced at the point dshown in FIGS. 6A and 6B where the display quantity at the immediatelyproceding time point coincides with the instant scan line number. Forimplementing the above conditions, the input J₂ may be connected to anOR-gate in FIG. 4A.

Further, although the relation between the instant display quantity andthe one at the immediately preceding display time point has been takeninto consideration in the foregoing description, it will be appreciatedthat similar effect can be attained starting from the relation betweenthe instant display quantity and the one at the immediately followingtime point. In this case, circuit arrangement shown in FIG. 3 is soarranged that the display quantity at the time point immediatelyfollowing the instant time point is read out from the trend data memory510n. To this end, the first comparator circuit 530n will receive at theinput terminal E the display quantity at the immediately following timepoint, while the input terminal E' of the second comparator circuit 540nis applied with the quantity to be displayed at the instant time point.In this conjunction, the bright spot control logic circuit may beconstructed such that the video signal K can be produced at the outputterminal K, when the following input conditions are logically satisfied:

    K=I.sub.1 ×J.sub.3 +J.sub.2 +I.sub.3 ×J.sub.1  (2)

An exemplary circuit for implementing the above conditions isillustrated in FIG. 4B. Under these conditions, the bright spot controllogic circuit will produce the video signal output 501n, provided that

(d) the quantity to be displayed at the instant time point coincideswith the instant scan line number (i.e. when the input signal "1" ispresent at the terminal J₂),

(e) the instant scan line number is smaller than the quantity to bedisplayed at the instant time point and greater than the displayquantity at the immediately following time point (i.e. when the inputsignal "1" is present at both the terminals I₁ and J₃), or

(f) the scan line number is greater than the quantity to be displayed atthe instant time point and smaller than the immediately followingdisplay quantity (i.e. the input signal "1" is present at both theterminals I₃ and J₁).

Consequently, assuming by way of example that the quantity to bedisplayed at the instant time point t_(n) is equal to n+4, while thequantity to be displayed at the immediately following time point t_(n+1)is equal to n, then the bright spot will be generated at the end point awhere the instant display quantity coincides with the instant scan linenumber, points e where the associated scan line numbers are smaller thanthe instant display quantity and greater than the immediately followingdisplay quantity, and points f where the associated scan line numbersare greater than the instant display quantity and smaller than the oneto be displayed at the immediately following time point, as shown inFIGS. 6C and 6D.

In this manner, clear display of the trend graph can be accomplishedthrough the interpolation of the bright spots, even if the trend dataundergoes rapid variation, as is shown by the trend lines n and o inFIG. 5.

FIG. 7 shows another embodiment of the invention which differs from theone shown in FIG. 3 in that a latch circuit 570n for fetching thereinthe result of comparison from the first comparator circuit 530n isprovided in place of the second comparator circuit 540n and the datalatch circuit 550n. The comparison latch circuit 570n has inputterminals D₁, D₂ and D₃ connected to the output terminals G₁, G₂ and G₃of the first comparator circuit 530n and a trigger terminal T adapted tobe applied with the dot timing pulse 401 for every display time point.Accordingly, in a case of reading out the display quantity correspondingto the instant time point from the data memory 510n, there is producedat the output terminals Q₁, Q₂ and Q₃ the output signals representingthe results of comparison between the trend quantity to be displayed atthe time point immediately preceding the instant time point and theinstant scan line number. These outputs Q₁ to Q₃ are thus absolutelyidentical with the output signals H₁ to H₃ from the second comparisoncircuit 540n shown in FIG. 3 and applied to the bright spot controllogic circuit 560 to produce the video signal 501n.

In this embodiment, it is also possible as in the case of the embodimentof FIG. 3 to arrange the circuits so as to read out the display quantitycorresponding to the immediately following time point from the datamemory. The circuit configuration shown in FIG. 7 allows the amount ofhardware to be reduced as compared with the circuit shown in FIG. 3.Particularly, IC can be reduced for every trend graph adapter, reducingthe total number of IC's used when a plurality of the trend graphadapters are employed for producing a corresponding number of trendcurves on a single screen.

It will be now appreciated that the invention has provided a trend graphdisplay system which is capable of producing a clear image of trendgraphs independently from rapid variations in the trend data with asimplified circuit arrangement.

We claim:
 1. A trend graph display system including a cathode ray tubehaving a viewing screen on which time points represented by a pluralityof points serially related in time spaced from one another with apredetermined time interval are displayed in the raster direction, whiledisplay quantities of trend graphs are displayed as functions of saidtime points in the direction perpendicular to said raster direction byrespective pluralities of point s spaced from one another with apredetermined interval, each point of said respective plurality locatedin a different raster scan from the remaining points of said respectiveplurality, said cathode ray tube being so adapted that a raster scanningis effected by sweeping said viewing screen with a scanning pointwhereby the scanning lines thus produced are correlated to said spacedpoints representing said display quantities, further including;memorymeans for storing said display quantities of said trend graphcorresponding to said serially related time points; reading means forsequentially reading out said display quantities from said memory meansin the order of said serially related time points; and comparison meansfor comparing the display quantity read out from said memory means witha display quantity represented by an instant scan line, said comparisonmeans comparing the display quantity read out from said memory meanscorresponding to an instant time point corresponding to an instantposition of the scanning spot of said instant scan line on said viewingscreen with a display quantity represented by an instant scan line,thereby to supply a video signal to said cathode ray tube uponcoincidence being found in said comparison, said comparison meansfurther including means for supplying said video signal to said cathoderay tube when said instant display quantity represented by said instantscan line lies in a range delimited by the display quantity at saidinstant time point and the display quantity at the selected one timepoint of the time points immediately preceding and following saidinstant time point.
 2. A trend graph display system according to claim1, wherein said reading means is adapted to output read-out signals forsequentially reading out said display quantities from said memory meansand a scan line signal representing the display quantity to be displayedby the instant scan line, while said memory means is adapted to outputsaid display quantities sequentially in the order of said display timepoints in response to said read-out signals, and wherein said comparisonmeans includes a latch circuit for producing said display quantity readout from said memory means with a delay equal to the duration of saidpredetermined time interval, a first comparator circuit for comparingthe display quantity read out from said memory means and said scan linesignal, a second comparator circuit for comparing said delayed displayquantity output from said latch circuit with said scan line signal, anda video signal generating circuit for producing said video signal independence on the results of comparisons at said first and secondcomparator circuits.
 3. A trend graph display system according to claim1, wherein said reading means is adapted to output read-out signals forsequentially reading out said display quantities from said memory meansand a scan line signal representing the display quantity to be displayedby the instant scan line, while said memory means is adapted to outputsaid display quantities sequentially in the order of said display timepoints in response to said read-out signal, wherein said comparisonmeans includes a comparator circuit for comparing said display quantityread out from said memory means with said scan line signal, a latchcircuit for producing the result of comparison at said comparatorcircuit with a delay corresponding to the duration of said predeterminedtime interval, and a video signal generator circuit for producing saidvideo signal in dependence on said result of comparison and the outputstate of said latch circuit.
 4. A trend graph display system accordingto claim 2, wherein said memory means is adapted to output sequentiallysaid display quantities each corresponding to said instant time pointcorresponding to the instant position of said scan spot on said viewingscreen in response to said read-out signal.
 5. A trend graph displaysystem according to claim 2, wherein said memory means is adapted tooutput sequentially said display quantities each corresponding to a timepoint following immediately the instant time point corresponding to theinstant position of said scan spot on said viewing screen in response tosaid read-out signal.
 6. A trend graph display system according to claim3, wherein said memory means is adapted to output sequentially saiddisplay quantities each corresponding to said instant time pointcorresponding to the instant position of said scan spot on said viewingscreen in response to said read-out signal.
 7. A trend graph displaysystem according to claim 3, wherein said memory means is adapted tooutput sequentially said display quantities each corresponding to a timepoint following immediately the instant time point corresponding to theinstant position of said scan spot on said viewing screen in response tosaid read-out signal.
 8. A trend graph display system according to claim4, wherein said first comparator circuit is adapted to produce a first,a second and a third signal, respectively, in response to magnitudes ofsaid scan line signal greater than, equal to and smaller than saiddisplay quantity at said instant time point, while said secondcomparator circuit is adapted to produce a fourth, a fifth and a sixthsignal, respectively, in response to magnitudes of said scan line signalgreater than, equal to and smaller than said display quantity at a timepoint immediately preceding said instant time point, and wherein saidvideo signal generator circuit is adapted to output the video signalwhen said second signal is present, when both of said first and sixthsignal are present or when both of said third and fourth signals arepresent.
 9. A trend graph display system according to claim 5, whereinsaid first comparator circuit is adapted to produce a first, a secondand a third signal, respectively, in response to magnitudes of said scanline signal greater than, equal to and smaller than said displayquantity at a time point immediately following said instant time point,while said second comparator circuit is adapted to produce a fourth, afifth and a sixth signal, respectively, in response to magnitudes ofsaid scan line signal greater than, equal to and smaller than saiddisplay quantity at said instant time point, and wherein said videosignal generator circuit is adapted to output the video signal when saidfifth signal is present, when both of said first and sixth signal arepresent or when both of said third and fourth signals are present.
 10. Atrend graph display system according to claim 8, wherein said videosignal generator circuit is adapted to further output the video signalwhen said fifth signal is present.
 11. A trend graph display systemaccording to claim 9, wherein said video signal generator circuit isadapted to further output the video signal when said second signal ispresent.
 12. A trend graph display system according to claim 6, whereinsaid comparator circuit is adapted to produce a first, a second and athird signal, respectively, in response to magnitudes of said scan linesignal greater than, equal to and smaller than said display quantity atsaid instant time point, while said latch circuit is adapted to delaysaid first, second and third signals by a time corresponding to theduration of said predetermined time interval thereby outputting them asa fourth, a fifth and a sixth signal respectively, and wherein saidvideo signal generator circuit is adapted to output the video signalwhen said second signal is present, when both of said first and sixthsignals are present, or when both of said third and fourth signals arepresent.
 13. A trend graph display system according to claim 7, whereinsaid comparator circuit is adapted to produce a first, a second and athird signal, respectively, in response to magnitudes of said scan linesignal greater than, equal to and smaller than said display quantity ata time point immediately following said instant time point, while saidlatch circuit is adapted to delay said first, second and third signalsby a time corresponding to the duration of said predetermined timeinterval thereby outputting them as a fourth, a fifth and a sixth signalrespectively, and wherein said video signal generator circuit is adaptedto output the video signal when said fifth signal is present, when bothof said first and sixth signals are present, or when both of said thirdand fourth signals are present.
 14. A trend graph display systemaccording to claim 12, wherein said video signal generator circuit isadapted to further output the video signal when said fifth signal ispresent.
 15. A trend graph display system according to claim 13, whereinsaid video signal generator circuit is adapted to further output thevideo signal when said second signal is present.